In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. While a number of light sources are used in the lithography for resist pattern formation, photolithography using ArF excimer laser (193 nm) has been under active investigation over a decade. The full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the manufacture of next 45-nm node devices, the early introduction of ArF immersion lithography was advocated (see Proc. SPIE Vol. 4690 xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, design of a lens having a numerical aperture (NA) of 1.0 or greater is possible in principle. Proposal of a catadioptric system accelerated the lens design to a NA of 1.0 or greater. A combination of a lens having NA of 1.2 or greater with strong resolution enhancement technology suggests a way to the 45-nm node (see Proc. SPIE Vol. 5040, p 724, 2003). Efforts have also been made to develop lenses of NA 1.35.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line edge roughness (LER, LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
The water immersion lithography using a NA 1.35 lens achieves an ultimate resolution of 40 to 38 nm at the maximum NA, but cannot reach 32 nm. Efforts have been made to develop higher refractive index materials in order to further increase NA. It is the minimum refractive index among projection lens, liquid, and resist film that determines the NA limit of lenses. In the case of water immersion, the refractive index of water is the lowest in comparison with the projection lens (refractive index 1.5 for synthetic quartz) and the resist film (refractive index 1.7 for prior art methacrylate-based film). Thus the NA of projection lens is determined by the refractive index of water. Recent efforts succeeded in developing a highly transparent liquid having a refractive index of 1.65. In this situation, the refractive index of projection lens made of synthetic quartz is the lowest, suggesting a need to develop a projection lens material with a higher refractive index. LuAG (lutetium aluminum garnet Lu3Al5O12) having a refractive index of at least 2 is the most promising material, but has the problems of birefringence and noticeable absorption. Even if a projection lens material with a refractive index of 1.8 or greater is developed, the liquid with a refractive index of 1.65 limits the NA to 1.55 at most, failing in resolution of 32 nm. For resolution of 32 nm, a liquid with a refractive index of 1.8 or greater is necessary. Such a liquid material has not been discovered because a tradeoff between absorption and refractive index is recognized in the art. In the case of alkane compounds, bridged cyclic compounds are preferred to linear ones in order to increase the refractive index, but the cyclic compounds undesirably have too high a viscosity to follow high-speed scanning on the exposure tool stage. If a liquid with a refractive index of 1.8 is developed, then the component having the lowest refractive index is the resist film, suggesting a need to increase the refractive index of a resist film to 1.8 or higher.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern in spaces of the first pattern. See Proc. SPIE Vol. 5992, 59921Q-1-16 (2005). A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure pattern, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at a half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a second photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
While the former process requires two applications of hard mask, the latter process uses only one layer of hard mask, but requires to form a trench pattern which is difficult to resolve as compared with the line pattern. The latter process includes the use of a negative resist material in forming the trench pattern. This allows for use of high contrast light as in the formation of lines as a positive pattern. However, since the negative resist material has a lower dissolution contrast than the positive resist material, a comparison of the formation of lines from the positive resist material with the formation of a trench pattern of the same size from the negative resist material reveals that the resolution achieved with the negative resist material is lower. After a wide trench pattern is formed from the positive resist material by the latter process, there may be applied a thermal flow method of heating the substrate for shrinkage of the trench pattern, or a RELACS method of coating a water-soluble film on the trench pattern as developed and heating to thicken the resist for achieving shrinkage of the trench pattern. These have the drawbacks that the proximity bias is degraded and the process is further complicated, leading to reduced throughputs.
Both the former and latter processes require two etchings for substrate processing, leaving the issues of a reduced throughput and deformation and misregistration of the pattern by two etchings.
One method that proceeds with a single etching is by using a negative resist material in a first exposure and a positive resist material in a second exposure. Another method is by using a positive resist material in a first exposure and a negative resist material in an alcohol that does not dissolve away the positive resist material in a second exposure. Since negative resist materials with low resolution are used, these methods entail degradation of resolution (see JP-A 2008-078220).
The critical issue associated with double patterning is an overlay accuracy between first and second patterns. Since the magnitude of misregistration is reflected by a variation of line size, an attempt to form 32-nm lines at an accuracy of 10%, for example, requires an overlay accuracy within 3.2 nm. Since currently available scanners have an overlay accuracy of the order of 8 nm, a significant improvement in accuracy is necessary.
Now under investigation is the resist pattern freezing technology involving forming a first resist pattern on a substrate, taking any suitable means for inactivating the resist pattern to a second resist process, applying a second resist thereon, and forming a second resist pattern in space portions of the first resist pattern. With this freezing technology, etching of the substrate is required only once, leading to improved throughputs and avoiding the problems of pattern deformation and misregistration due to stresses in the hard mask during etching.
With respect to the freezing technology, one basic idea is proposed in WO 2008/059440. Known variants of the freezing technology include thermal insolubilization (Proc. SPIE Vol. 6923, p69230G (2008)); coating of a cover film and thermal insolubilization (Proc. SPIE Vol. 6923, p69230H (2008)); insolubilization by illumination of light having an extremely short wavelength, for example, 172 nm wavelength (Proc. SPIE Vol. 6923, p692321 (2008)); insolubilization by ion implantation (Proc. SPIE Vol. 6923, p692322 (2008)); insolubilization through formation of thin-film oxide film by CVD; insolubilization by light illumination and special gas treatment (Proc. SPIE Vol. 6923, p69233C1 (2008)); insolubilization of a resist pattern by treatment of resist pattern surface with a metal alkoxide or metal halide (e.g., titanium, zirconium or aluminum) or an isocyanate-containing silane compound (JP-A 2008-033174); insolubilization of a resist pattern by coating its surface with a water-soluble resin and a water-soluble crosslinker (JP-A 2008-083537); insolubilization by ethylene diamine gas and baking (J. Photopolym. Sci. Technol., Vol. 21, No. 5, p 655 (2008)); insolubilization by coating of an amine-containing solution and hard-baking for crosslinking (WO 2008/070060); and insolubilization of resist pattern by treatment with a mixture of a polar resin containing amide or analogous groups and a crosslinker (WO 2008/114644).
JP-A 2008-192774 discloses a method including insolubilizing a first resist pattern by application of radiation and heat, coating the insolubilized pattern with a resist solution comprising a base polymer comprising recurring units having hexafluoroalcohol groups and acid labile groups in an alcohol solvent, and forming a second resist pattern therefrom.
These insolubilizing techniques have many problems including deformations (film-slimming, size-narrowing and size-widening) of the first resist pattern during the steps of first resist pattern inactivation and second patterning. It would be desirable to have a material and process capable of retaining the first resist pattern without deformation until the end of second patterning.